(1) Field of the Invention
The present invention relates to a microporous drawn film excellent in air permeability and moisture permeability and a process for the preparation thereof. The microporous drawn film of the present invention is preferably used as a material for a separator of a battery, a capacitor, a waste water disposal, a purification of water, an artificial leather, an artificial paper, a heat-insulating packaging film, a moisture absorbent packaging film, a bandage, an operating gown, a mask, a back-sheet of a plaster, a sterile packaging film and the like.
(2) Description of the Prior Art
The preparation of a porous film has been tried by incorporating a filler having a small particle size into polypropylene, forming a film by melt molding and extracting the filler from the film. This process, however, is defective in that if a large amount of the filler is incorporated in order to increase the porosity, because of secondary aggregation of the filler it is difficult to form a uniform porous structure, or the extraction efficiency is low and the porosity is insufficient.
Various improvements have been made for eliminating this disadvantage. For example, Japanese Patent Publication No. 32171/83 proposes a process for the preparation of a porous film, which comprises melt-forming a film from a composition comprising 10 to 60% by volume of a polyolefin resin having a weight average molecular weight lower than 300,000 and a number average molecular weight of at least 15,000, 7 to 42% by volume of an inorganic fine powder and 30 to 75% by volume of an organic liquid and extracting the organic liquid from the formed film. According to this process, a practically applicable porous film can be obtained if polyethylene is used as the resin, but improvements are desired in strength, porosity, pore size uniformity and heat resistance of the obtained film. Moreover, if polypropylene is used in the above-mentioned process in order to impart inherent excellent properties of polypropylene to the resulting film, the film-forming operation cannot be performed conveniently and a porous film composed substantially of polypropylene cannot be obtained.
Furthermore, there is known a process in which pores are formed by stretching an inorganic filler-containing film (see Japanese Patent Application Laid-Open Specification No. 119970/74). This process is advantageous in that a required porosity can be attained without extracting a filler or process oil and the obtained film is satisfactory in strength and heat resistance. However, if a fine filler having a particle size smaller than 1.mu. is used, secondary aggregation cannot be prevented. For example, requirements for a separator of a battery or a capacitor, such as a maximum pore size smaller than 1.mu. and good air permeability, good moisture permeability and good pore size uniformity, cannot be satisfied according to this process.
U.S. Pat. No. 4,613,643 to Nakamura, one of us, et al. discloses a process in which a film of an elastomer containing an inorganic filler is stretched to form pores. According to this process, an intended porous film is obtained when the resin component contains an elastomer, but if polypropylene is used as the resin component, it is difficult to obtain a porous polypropylene film having a maximum pore size smaller than 1.mu. and a large porosity.